Media Gateway Interconnect Routing in a Softswitch Environment

ABSTRACT

Calls are routed in a distributed mobile switching center environment in response to receiving a call to be routed from a first switching mode ( 310 ( 1 ) to a second switching node ( 3101 ( 2 )). An inter-switch mute for the call from the first switching node to the second switching node is identified. The inter-switch route includes an intermediate switching node ( 305 ) having multiple media gateway ( 320 ). An intra-switch route within the intermediate switching node is selected from multiple possible intra-switch routes for use in establishing the inter-switch route. Each of the intra-switch routes includes one or more interconnections ( 330 ) between media gateways. The call is connected using the identified inter-switch route and the selected intra-switch route.

REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of co-pendingprovisional application serial number 60/564,091, filed Apr. 21, 2004,which is incorporated herein by reference.

TECHNICAL FIELD

This description relates to telecommunications, and more particularly torouting calls in a distributed mobile switching center environment usingmedia gateway interconnections.

BACKGROUND

Conventional cellular telecommunications networks include mobileswitching centers (MSCs) each operating to route calls between basestations that include radio equipment for serving one or more cells inthe cellular network and other MSCs or public switched telephonenetworks (PSTNs). Conventional MSCs handle both bearer traffic fortransporting user information (e.g., voice or other data in anintegrated services digital network (ISDN)) and signaling traffic forestablishing and controlling call connections and managing the cellularnetwork.

SUMMARY

Distributed MSCs (e.g., as can be used with 3GPP, Release 4) perform thesame general functions as a conventional MSC but include an MSC serverfor handling signaling traffic and multiple media gateways for handlingbearer traffic. The MSC server includes the intelligence and complexityof the distributed MSC, and the media gateways are controlled by the MSCserver. The media gateways can be geographically distributed, can eachcontrol multiple base stations, and serve to route bearer traffic undercontrol of the MSC server.

In one general aspect, a call to be routed from a first switching nodeto a second switching node is received, and an inter-switch route forthe call from the first switching node to the second switching node isidentified. The inter-switch route includes an intermediate switchingnode that includes multiple media gateways. An intra-switch route withinthe intermediate switching node is selected from multiple possibleintra-switch routes for establishing the inter-switch route. Each of theintra-switch routes includes one or more interconnections between mediagateways. The call is connected using the identified inter-switch routeand the selected intra-switch route.

Implementations can include one or more of the following features. Theintra-switch route is selected according to a selection algorithm. Theinter-switch route is identified by selecting the inter-switch routefrom a list of inter-switch routes for use with calls to be routed fromthe first switching node to the second switching node. A receiveddirectory number for use in selecting the list of inter-switch routesfor the call is translated. An availability of the identifiedinter-switch route is determined, and an availability of theinterconnections of the selected intra-switch route is also determined.An unavailability of a candidate intra-switch route is determined basedon an unavailability of one or more interconnections, and theintra-switch route is selected in response to determining theunavailability of the candidate intra-switch route. An intra-switchroute is selected from a list of intra-switch routes associated with theidentified inter-switch route. The intermediate switching node isassociated with multiple trunks, at least one of which connects theintermediate switching node to the first switching node. One or more ofthe trunks also connects the intermediate switching node to the secondswitching node, and each interconnection is an interconnection betweenmedia gateways. The media gateways handle bearer traffic, and each ofthe media gateways operate under control of a server using signalingtraffic associated with the bearer traffic.

In another general aspect, a telecommunications system includes a firstswitching node and a second switching node that is a distributed mobileswitching center. One or more media gateways in the distributed mobileswitching center have a trunk connection with the first switching node.A server of the distributed mobile switching center controls routing forthe media gateways. The server selects, for a call connection, anintra-switch route that includes a first media gateway, a second mediagateway, and one or more interconnections for connecting the first mediagateway and the second media gateway. The intra-switch route is selectedfrom multiple possible intra-switch routes designated for serving callconnections routed through the first switching node and the secondswitching node.

Implementations can include one or more of the following features. Theintra-switch routes are included in an interconnection route listdesignating a set of possible intra-switch routes from the first mediagateway to the second media gateway. The server selects the intra-switchroute based on a selection algorithm defining a sequence in which theintra-switch routes are selected from the interconnection route list.The selection algorithm includes one or more restrictions forselectively preventing use of an intra-switch route based on datarelating to the call connection. The first switching node is adestination switching node, and the server of the second switching nodeidentifies an external route from a list of external routes between thefirst switching node and the second switching node. The first switchingnode is an origination switching node for the call connection, which isdestined for a third switching node.

The first switching node selects an inter-switch route, which includesthe second switching node, from a list of routes for routing callconnections from the first switching node to the third switching node,and the server selects the intra-switch route in response to a messagefrom the first switching node. The server identifies an unavailabilityof a circuit in an initially selected intra-switch route and, inresponse to identifying the unavailability, selects an alternateintra-switch route that uses a different circuit on the initiallyselected intra-switch route or a different intra-switch route having oneor more alternate interconnections. The one or more interconnectionsinclude circuits for handling bearer traffic in one or both directions,and the interconnections have an associated continuity testingattribute. The server handles signaling traffic for the distributedmobile switching center, and the media gateways handle bearer trafficfor the distributed mobile switching center.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a telecommunications network that includesa distributed mobile switching center (MSC).

FIG. 2 is a schematic diagram of a routing methodology for selectingcall routes.

FIG. 3 is a block diagram of a network that includes a distributed MSC.

FIG. 4 is a flow diagram of a route selection process

FIG. 5 is a block diagram of a continuity testing procedure forautomatically testing whether circuits for connecting switches are inservice and not experiencing problems or excessive delays.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a telecommunications network 100 thatincludes a distributed mobile switching center (MSC) 105. Thedistributed MSC 105 includes an MSC server 110 that controls multiplemedia gateways (MGWs) 115(1) . . . 115(n), which are connected byinterconnections 120 through which bearer traffic can be routed betweendifferent media gateways 115. The media gateways 115 can begeographically distributed such that each media. gateway 115 isassociated with a number of base stations and/or base stationcontrollers 125 that serve different geographical areas. One or more ofthe media gateways 115 interface with a PSTN 130 or other switchingnode, such as a mobile switching center.

When a call is placed from a telephone 135 in a fixed network to amobile station 140 in a cellular network, the PSTN 130 routes the callthrough an originating trunk 145 to a media gateway 115(1) in thecellular network and sends signaling data to the MSC server 110 in, forexample, an ISDN user part (ISUP) message 150. The MSC server 110 sendssignaling data to a base station controller 125 serving an area in whichthe mobile station 140 is located for establishing a radio connection155 with the mobile station 140. In addition, the MSC server 110 directsthe media gateway 115(1) connected to the originating trunk 145 to routethe call through an interconnection 120 to another media gateway 115(2)that is capable of connecting with the base station controller 125, ifnecessary (i.e., if the originating media gateway 115(1) is notconnected to the base station controller 125). The MSC server 110 alsodirects the other media gateway 115(2) to route the call through aterminating trunk 160 to the base station controller 125 to establish acall connection between the telephone 135 and the mobile station 140.

FIG. 2 is a schematic diagram of a routing methodology 200 for selectingcall routes. When a call is received, the called number is input to anumber translation 205, in which the called number is translated. Usingthe translation, a route index 210 is identified based, for example, onthe called number, a time, an originating party, an originating trunk,and/or a circuit from which the call is received. The route index 210points to a route list 215 that includes a sequence of routing rules forrouting the call. The route list 215 can include a number of entries,such as trunk group, trunk group bundle, cause code, and the like. Eachentry can include data fields such as type, parameter, and out pulseindex. Based on the route list 215, a circuit selection algorithm 220 isused to assign a terminating circuit for the call, which results in, forexample, selection of a particular switching circuit or some otherfunction (e.g., an announcement if a circuit is unavailable).

FIG. 3 is a block diagram of a network 300 that includes a distributedMSC 305. The network 300 includes multiple switches 310(1), 310(2),310(3), and 310(4) external to the distributed MSC 305, each of whichcan be, for example, a switch in a PSTN, a conventional MSC, anotherdistributed MSC or soft switch, or a gateway MSC. The distributed MSC305 includes a call server 315 and a plurality of media gateways 320(1),320(2), 320(3), and 320(4). The call server 315 contains theintelligence and primary processing resources for the distributed MSC305. Accordingly, the call server 315 sends and receives signaling datato and from the external switches 310 and controls the switchingactivities of the media gateways 320. For example, call setup messagingis handled by the call server 315, and the call server 315 instructs themedia gateways 320 to route calls using particular circuits. Thus, thecall server 315 includes connections (not shown) with each of the mediagateways 320 (for sending control data) and with the external switches310 (for communicating signaling data).

The media gateways 320 handle bearer data under control of the callserver 315. The media gateways 320 can be geographically distributed,such that a first media gateway 320(1) of the distributed MSC 305 servesa first geographical area and a second media gateway 320(1) serves asecond geographical area, which are remote from one another and from thephysical location of the call server 315. In some cases, two or moremedia gateways 320 can be located in the same geographical area but canstill be remote from the call server 315 and/or other media gateways320.

To the external switches 310, the distributed MSC 305 generally has thecharacteristics of a conventional switch. In other words, the logicaland geographic separation of, including the division of functionsbetween, the call server 315 from the media gateways 320 within thedistributed MSC 305 is generally immaterial to the external switches.Thus, the distributed MSC 305 operates as a single switch from the pointof view of the external switches 310.

Each media gateway 320 has one or more trunk connections 325 with one ormore of the external switches. For example, the first media gateway320(1) includes a trunk connection 325 with switch A 310(1) and a trunkconnection 325 with switch B 310(2). In addition, each media gateway 320has one or more interconnection trunks 330 with other media gateways320. For example, the first media gateway 320(1) includes aninterconnection trunk 330 with the second media gateway 320(2) andanother interconnection trunk 330 with a third media gateway 320(3). Theinterconnection trunks 330 between media gateways can use time-divisionmultiplexing (TDM), internet protocol (IP), and/or asynchronous transfermode (ATM) for physically connecting the nodes. Each interconnectiontrunk 330 can be one-way (i.e., directional) connections or two-wayconnections and connects two media gateways 320. In an exampleimplementation, each interconnection trunk 330 supports about 16,000circuits. Generally, only one two-way or two one-way interconnectiontrunks 330 exist between each pair of media gateways 320.

In some possible implementations, only one route is defined between twomedia gateways 320 within the distributed MSC 305. For example, thefirst media gateway 320(1) can route calls to a fourth media gateway320(4) using TDM circuits contained in a direct interconnection trunk330 between the first media gateway 320(1) and a fourth media gateway320(4). In such an implementation, if the call server 315 receives asignaling message indicating that a call is to be routed from switch A310(1) to switch C 310(3), the call server 315 determines that the callis to be received from switch A 310(1) at the first media gateway320(1), routed through an interconnection trunk 330 (i.e., ICT-1-4,where “ICT” stands for interconnection trunk and “1-4” indicates thatthe interconnection trunk 330 is between the first media gateway 320(1)and the fourth media gateway 320(4)), and sent from the fourth mediagateway 320(4) to switch C 310(3). If the call server 315 determinesthat the circuits between the first media gateway 320(1) and the fourthmedia gateway 320(4) (i.e., ICT-1-4) are unavailable (e.g., due to outof service and/or busy circuits), the call server 315 has to reject thecall, which may then be connected through an alternate route thatbypasses the distributed MSC 305.

In other implementations, multiple routes are defined within thedistributed MSC 305 for connecting one particular media gateway 320(e.g., the first media gateway 320(1)) to another media gateway 320(e.g., the second media gateway 320(2)). By defining multiple routes(e.g., indirect routes) between each pair of media gateways 320, thedistributed MSC 305 can provide transmission redundancy. Accordingly, ifone intra-switch route is unavailable, a different intra-switch route(i.e., using a different combination of interconnection trunks 330) canbe selected, and the call does not need to be routed through a differentinter-switch route to bypass the distributed MSC 305.

Among other things, handling the call within the distributed MSC 305 canbe desirable in some situations, such as when the geographicdistribution of the media gateways 320 within the distribute MSC 305allow an operator to avoid long distance fees that may be associatedwith re-routing the call so as to bypass the distributed MSC 305. Forexample, a bypass of the distributed MSC 305 may require the call to berouted through an inter-exchange carrier network, the use of whichsubjects the call to charges that can be avoided by routing the callbetween geographically distributed media gateways 320 within thedistributed MSC 305.

The multiple routes between media gateways 320 (i.e., intra-switchroutes) can be defined in an interconnection route list (ICRL). Forexample, when a call arrives at the first media gateway 320(1) of thedistributed MSC 305 from external switch A 310(1), the call server 315(in response to a message from switch A 310(1) requesting that the callserver 315 set up the call) may identify a outbound trunk 325, withrespect to the distributed MSC 305, from the second media gateway 320(1)to external switch B 310(2). As a result, a route from the first mediagateway 320(1) to the second media gateway 320(2) needs to beidentified. An interconnection route list for identifying connectionsbetween the first media gateway 320(1) and the second media gateway320(2) can be:

ICRL-1-2={ICT-1-2, {ICT-1-4, ICT 4-2}, {ICT-1-3, ICT-3-2}},

which defines a set of possible interconnection routes, including adirect route (i.e., ICT-1-2), a route that passes through the fourthmedia gateway 320(4) (i.e., {ICT-1-4, ICT-4-2}, and a route that passesthrough the third media gateway 320(3) (i.e., {ICT-1-3, ICT-3-2}). Thus,an interconnection route list is a collection of routes that aredesignated for connecting one media gateway 320 to another media gateway320.

In some cases, some of the interconnection routes can be subject torestrictions or preferences relating to data associated with the call.For example, an operator of the distributed MSC 305 may want to savevaluable interconnection resources for its own customers. Thus, someinterconnection routes may be reserved for the operator's customers.Other data associated with a call (e.g., the calling party's number, thecalled party's number, whether the call originates in another provider'snetwork, the identity of the calling party's and/or called party'sprovider, the incoming or outgoing trunk, and the like) can also be usedfor implementing restrictions or preferences.

In addition, the interconnection routes can be selected using aselection algorithm that defines, for example, a sequence in whichinterconnection routes are selected from the interconnection route list.The selection algorithm can define a sequential selection (e.g., selectthe first interconnection route in the list until it is no longeravailable, then move to the next interconnection route, and so on), apercentage approach (e.g., for the first one hundred calls, select afirst interconnection route until sixty calls are using the firstinterconnection route, select a second interconnection route untilthirty calls are using the second interconnection route, select a thirdinterconnection route until ten calls are using the thirdinterconnection route, and repeat for each additional one hundredcalls), or a most idle approach (e.g., select the interconnection routewith the most idle interconnection trunks 330). Circuits within aninterconnection route can also be selected according to a circuitselection algorithm.

In addition to the interconnection route list, and external route listcan also be defined. The external route list can be used to identify alist of external switches 310 to which a call can be routed to the nextswitch in an overall inter-switch route. For example, if a calloriginates at switch A 310(1) and, based on a number translation, it isdetermined that the call is destined for switch B 310(2), switch A310(1) may use a route table to determine that the call needs to be sentto the distributed MSC 305. The call server 315, in turn, determines, oris informed by switch A 310(1), that the call needs to be routed toswitch B 310(2). The call server 315 can refer to an external route listassociated with routing calls to switch B 310(2). The external routelist for switch B 310(2) can include:

RT-1-B, RT-3-D, RT-2-B,

indicating that possible routes from the distributed MSC 305 that can beused to reach switch B 310(2) include a route (i.e., RT-1-B) through thefirst media gateway 320(1), a route (i.e., RT-3-D) through the thirdmedia gateway 320(3) (and subsequently through a trunk between switch D310(4) and switch B 310(2)), and a route (i.e., RT-2-B) through thesecond media gateway 320(2). The call server 315 can select an externalroute from the external route list and can used at least some of thesame techniques (e.g., restrictions and preferences) as described inconnection with use of the interconnection route list.

FIG. 4 is a flow diagram of a route selection process 400. A dialednumber is received and translated (405). The translation analyzes thereceived dialed number by, for example, extracting the area code,determining the type of number (e.g., a special number, such as *33 or911), determining if it is long distance or local, and the like. Thetranslation is used to refer to a route index (410), in which a routelist corresponding to the translated number is identified (415). Theroute list defines routes that go from an origination switch (e.g.,switch A 310(1)) to a termination switch (i.e., a destination switch,such as switch B 310(2)). A determination is made as to whether a routecandidate from the route list is available (420). The determination mayinclude determining if circuits for the trunks 325 necessary toestablish a connection using the route candidate are idle and inservice. If not, no route is available (425), and the call is rejected,in which case the call may need to be routed through an alternateswitch.

If a route candidate is available (e.g., RT-2-B, in the external routelist example above), a determination is made (430) as to whether aninterconnection route necessary to establish the selected candidateroute (e.g., RT-2-B) is available. This latter determination can includeexamining each interconnection route an interconnection route list(e.g., ICRL-1-2, in the interconnection route list example above) in anorder dictated by a selection algorithm. If no interconnection routesfrom the interconnection route list are available, a determination ismade as to whether another route candidate (e.g., RT-3-D) from the routelist is available (420). If so, a determination is made (430) as towhether an interconnection route necessary to establish the newlyselected candidate route (e.g., RT-3-D) is available.

This process repeats until an available route list and an availableinterconnection route are found or it is determined (at 420) that nofurther route lists are available. Once an available route list and anavailable interconnection route are found, a final route that includesan external route and an interconnection route is established (435), andthe call can be connected. In some cases, such as if the route candidateselected is RT-1-B, no interconnection trunk 330 is necessary. Bychecking for an available interconnection route, the process 400 canensure that an interconnection route is available before allocatingresources for the call.

FIG. 5 is a block diagram of a continuity testing procedure 500 forautomatically testing whether circuits for connecting switches are inservice and not experiencing problems or excessive delays. Continuitytesting (COT) can be triggered in a call server 505 by an external COTrequest from an incoming call or as a result of an internal periodicsetting (e.g., to ensure availability of the circuits). When a call isplaced that is to be routed from an originating switch 510, across afirst trunk 515, through a first media gateway 520, across aninterconnection trunk 525, through a second media gateway 530, across asecond trunk 535, and to a terminating switch 540, a first initialaddress message (LAM) 545 is sent from the originating switch 510 to thecall server 505. The first IAM 545 includes a COT request. In accordancewith the COT request, a first COT test 555 can be performed between theoriginating switch 510 and the first media gateway 520 before sending asecond IAM 550 to the terminating switch 540.

A second COT test 560 can then be performed as part of theinterconnection route selection process. For example, the call server505 can signal the first media gateway 520 and the second media gateway530 (through signals 570) to perform the second COT test 560. If a COTfailure occurs in the second COT test 560, an alternate circuit or routecan be selected for the call. If an alternate media gateway route isselected, another inter-media gateway COT can be performed on thecircuits of the new media gateway route. The circuit that failed can bemarked as out of service. After a successful second COT test 560, asecond LAM 550 including a COT request is sent from the call server 505to the terminating switch 540. As a result, a third COT test 565 isperformed in accordance with typical ISUP COT procedure. Alarmconditions (e.g., critical, major, and minor) may be defined for whenout of service circuits surpass a predefined threshold level. Other testprocedures, such as a milliwatt test can also be used.

The invention and all of the functional operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structural meansdisclosed in this specification and structural equivalents thereof, orin combinations of them. The invention can be implemented as one or morecomputer program products, i.e., one or more computer programs tangiblyembodied in an information carrier, e.g., in a machine readable storagedevice or in a propagated signal, for execution by, or to control theoperation of, data processing apparatus, e.g., a programmable processor,a computer, or multiple computers. A computer program (also known as aprogram, software, software application, or code) can be written in anyform of programming language, including compiled or interpretedlanguages, and it can be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A computer program does notnecessarily correspond to a file. A program can be stored in a portionof a file that holds other programs or data, in a single file dedicatedto the program in question, or in multiple coordinated files (e.g.,files that store one or more modules, sub programs, or portions ofcode). A computer program can be deployed to be executed on one computeror on multiple computers at one site or distributed across multiplesites and interconnected by a communication network.

The processes and logic flows described in this specification, includingthe method steps of the invention, can be performed by one or moreprogrammable processors executing one or more computer programs toperform functions of the invention by operating-on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus of the invention can be implemented as, specialpurpose logic circuitry, e.g., an FPGA (field programmable gate array)or an ASIC (application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally,the processor will receive instructions and data from a read only memoryor a random access memory or both. The essential elements of a computerare a processor for executing instructions and one or more memorydevices for storing instructions and data. Generally, a computer willalso include, or be operatively coupled to receive data from or transferdata to, or both, one or more mass storage devices for storing data,e.g., magnetic, magneto optical disks, or optical disks. Informationcarriers suitable for embodying computer program instructions and datainclude all forms of non volatile memory, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto optical disks; and CD ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, the invention can be implementedon a computer having a display device, e.g., a CRT (cathode ray tube) orLCD (liquid crystal display) monitor, for displaying information to theuser and a keyboard and a pointing device, e.g., a mouse or a trackball,by which the user can provide input to the computer. Other kinds ofdevices can be used to provide for interaction with a user as well; forexample, feedback provided to the user can be any form of sensoryfeedback, e.g., visual feedback, auditory feedback, or tactile feedback;and input from the user can be received in any form, including acoustic,speech, or tactile input.

The invention can be implemented in a computing system that includes aback-end component, e.g., as a data server, or that includes amiddleware component, e.g., an application server, or that includes afront-end component, e.g., a client computer having a graphical userinterface or a Web browser through which a user can interact with animplementation of the invention, or any combination of such back-end,middleware, or front-end components. The components of the system can beinterconnected by any form or medium of digital data communication,e.g., a communication network. Examples of communication networksinclude a local area network (“LAN”) and a wide area network (“WAN”),e.g., the Internet.

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

1. A method for routing calls in a distributed mobile switching centerenvironment, the method comprising: receiving a call to be routed from afirst switching node to a second switching node; identifying aninter-switch route for the call from the first switching node to thesecond switching node, the inter-switch route including an intermediateswitching node, wherein the intermediate switching node includes aplurality of media gateways; selecting an intra-switch route within theintermediate switching node from a plurality of intra-switch routeswithin the intermediate switching node for establishing the inter-switchroute, each of the plurality of intra-switch routes including at leastone interconnection between media gateways; and connecting the callusing the identified inter-switch route and the selected intra-switchroute.
 2. The method of claim 1 wherein identifying the inter-switchroute comprises selecting the inter-switch route from a list ofinter-switch routes for use with calls to be routed from the firstswitching node to the second switching node.
 3. The method of claim 1further comprising: determining an availability of the identifiedinter-switch route; and determining an availability of the at least oneinterconnection of the selected intra-switch route.
 4. The method ofclaim 1 further comprising determining an unavailability of a candidateintra-switch route based on an unavailability of at least oneinterconnection, wherein selecting the intra-switch route is performedin response to determining the unavailability of the candidateintra-switch route.
 5. The method of claim 1 wherein selecting anintra-switch route comprises selecting the intra-switch route from alist of intra-switch routes associated with the identified inter-switchroute, the list of intra-switch routes including the plurality ofintra-switch routes.
 6. The method of claim 1 wherein the intermediateswitching node is associated with a plurality of trunks, at least one ofthe plurality of trunks connecting the intermediate switching node tothe first switching node and at least one of the plurality of trunksconnecting the intermediate switching node to the second switching node,and each interconnection comprising an interconnection between mediagateways.
 7. The method of claim 1 wherein selecting the intra-switchroute comprises selecting the intra-switch according to a selectionalgorithm.
 8. The method of claim 1 further comprising translating areceived directory number for the call, the received directory numberfor use in selecting the list of inter-switch routes.
 9. The method ofclaim 1 wherein the plurality of media gateways are operable to handlebearer traffic, each of the plurality of media gateways operating undercontrol of a server using signaling traffic associated with the bearertraffic.
 10. A telecommunications system comprising: a first switchingnode; and a second switching node comprising a distributed mobileswitching center including: a plurality of media gateways, at least onemedia gateway having a trunk connection with the first switching node;and a server operable to control routing for the plurality of mediagateways, the server operable to select, for a call connection, anintra-switch route including a first media gateway, a second mediagateway, and at least one interconnection for connecting the first mediagateway and the second media gateway, the intra-switch route selectedfrom a plurality of intra-switch routes designated for serving callconnections routed through the first switching node and the secondswitching node.
 11. The telecommunications system of claim 10 whereinthe plurality of intra-switch routes are included in an interconnectionroute list designating a set of possible intra-switch routes from thefirst media gateway to the second media gateway.
 12. Thetelecommunications system of claim 10 wherein the first switching nodecomprises a destination switching node, the server of the secondswitching node identifying an external route from a list of externalroutes between the first switching node and the second switching node.13. The telecommunications system of claim 10 wherein the firstswitching node comprises an origination switching node for the callconnection destined for a third switching node, the first switching nodeoperable to select an inter-switch route from a list of routes forrouting call connections from the first switching node to the thirdswitching node, the selected inter-switch route including the secondswitching node, and the server operable to select the intra-switch routein response to a message from the first switching node.
 14. Thetelecommunications system of claim 11 wherein the server selects theintra-switch route based on a selection algorithm defining a sequence inwhich the plurality of intra-switch routes are selected from theinterconnection route list.
 15. The telecommunications system of claim14 wherein the selection algorithm includes at least one restriction forselectively preventing use of an intra-switch route based on datarelating to the call connection.
 16. The telecommunications system ofclaim 10 wherein the server is operable to identify an unavailability ofa circuit in an initially selected intra-switch route and, in responseto identifying the unavailability, to select an alternate intra-switchroute and that uses a different circuit on the initially selectedintra-switch route or a different intra-switch route having at least onealternate interconnection.
 17. The telecommunications system of claim 10wherein the at least one interconnection comprises a plurality ofcircuits for handling bearer traffic in at least one direction, theinterconnection having an associated continuity testing attribute. 18.The telecommunications system of claim 10 wherein the server handlessignaling traffic for the distributed mobile switching center and theplurality of media gateways handle bearer traffic for the distributedmobile switching center.
 19. An article comprising a machine-readablemedium storing instructions for causing data processing apparatus to:receive data indicating a call to be routed from a first switching nodeto a second switching node through an intermediate switching node, theintermediate switching node having a plurality of geographicallydistributed media gateways connected by interconnections; identify afirst media gateway and a second media gateway of the plurality ofgeographically distributed media gateways for use in routing the callfrom the first switching node to the second switching node; identify aninterconnection route list corresponding to a routing between the firstmedia gateway and the second media gateway, the interconnection routelist having a plurality of different interconnection routes between thefirst media gateway and the second media gateway; determine anavailability of at least one of the different interconnection routes;select an available one of the different interconnection routes; andallocate the selected interconnection route for use in routing the callfrom the first switching node to the second switching node.
 20. Thearticle of claim 19 wherein the second media gateway is identified froman external route list having a plurality of external routes, eachexternal route providing a possible route between the intermediateswitching node and another switching node for used in routing the callfrom the first switching node to the second switching node.